Moon Phase Date Calculator
Enter a date to see its Moon phase, age and illumination.
Result
How it is calculated
The tool reads your date as midnight UTC and counts the days since a reference new moon (6 January 2000, 18:14 UTC). It divides by the mean synodic month, 29.530588853 days, and keeps the remainder as the Moon's age. The phase fraction is f = age / 29.530588853 and the illumination is (1 − cos(2π f)) / 2 × 100 percent. The phase name comes from splitting the cycle into eight equal parts centred on New Moon, First Quarter, Full Moon and Last Quarter, so each name covers about three and a half days. Dates before the reference work too. Because it uses the mean month, the result can differ from the true astronomical phase by up to about half a day or more: it is accurate enough for planning, not for exact event times. A date it cannot read shows no result.
Worked example
Enter 24 April 2024. At 00:00 UTC that date is 8,874.24 days after the reference new moon. Dividing by 29.5306 gives 300 full cycles plus a remainder, so the age is 15.06 days. The phase fraction is 15.06 / 29.5306 = 0.510, and the illumination is (1 − cos(2π × 0.510)) / 2 = 99.9%. That falls in the Full Moon band, so the phase shown is Full Moon.
Common mistakes
- Expecting the exact minute of a phase. The tool gives one value per date, based on the mean cycle, so the true new or full moon can be up to about half a day off.
- Ignoring time zones. The date is taken at 00:00 UTC, so for places far from UTC the Moon that evening will be further along than the figure shown.
- Reading a Full Moon name as 100% illumination every time. The name covers a window of about three and a half days, and the percentage is what shows how bright the Moon is.
- Typing an impossible date and trusting it. Use year-month-day, such as 2024-04-24. A date like 2024-02-30 is not rejected; it rolls over to 1 March, so check the day you typed.
- Using the result for eclipses or tide tables, which need the true Moon position rather than the mean cycle.
About this calculator
The Moon cycles through its phases over a synodic month — about 29.53059 days, the time it takes to return to the same phase as seen from Earth (slightly longer than its 27.32-day orbital period because Earth itself moves around the Sun). This calculator finds a date's position within that cycle by measuring elapsed days since a known reference new moon, then reduces that count modulo the synodic month length to get the Moon's age in days and its phase name (New, Waxing Crescent, First Quarter, Waxing Gibbous, Full, Waning Gibbous, Last Quarter, Waning Crescent).
From the phase angle, illumination percentage is derived from how much of the Moon's visible disk is lit — roughly (1 − cos(phase angle))/2 — which is why illumination peaks near 100% at Full Moon and bottoms out near 0% at New Moon. This kind of calculation underlies lunar calendars, tide prediction, astrophotography planning, and religious observances (like the Islamic and Hebrew calendars) that are tied directly to the Moon's visible phase rather than the solar year.
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